Personally, I think I’d lean towards Pirates being comparatively balanced axe-users, Brigands being way in the direction of huge HP and Strength but bad Skill and mediocre other stats, and Fighters falling in between those two.
I think that really, the bottom line is Pirates and Brigands are both sort of… “Flavor classes”, for lack of a better term. They were implemented with the intention of them being essentially Fighters with some slight ability differences based on where they’d show up as enemies, and the main differences being cosmetic. That’s not to say that it’s not possible to differentiate them, but they are all infantry axefighters at the end of the day, and there’s only so much you can change that before they just become unrelated classes borrowing the names of the Fighter/Pirate/Brigand trio.